1. Field of the Invention
The present invention relates to a method and a device for the detection, by phase analysis, of the garbling of pulses received by a secondary radar. It can be applied notably to secondary radars receiving pulse responses from several aircraft at a time, and can be applied especially when the working frequencies of their transponders are very close to one another.
With air traffic becoming increasingly dense, the pulses sent out by the transponders of aircraft towards secondary radars on the ground get jumbled, creating phenomena of pulse-garbling. This garbling induces false codes in the reception and processing circuits of the secondary radars, which prevents the identifying of the aircraft or of their positions, and thereby leads to serious consequences for air traffic safety.
The presence of a pulse of a secondary response sent out by an aircraft is generally detected by means of a signal referenced Q.SIGMA., the amplitude of which is half that of the signal received by the sum channel of the antenna of the secondary radar. The processing of the secondary radar makes use of this type of information only, firstly in order to detect a response, especially the two known and standardized pulses F1 and F2 which are separated by 20.3 .mu.s and sandwich the response and, secondly, to detect the code of the response having the shape of a sequence of pulses. Consequently, all the secondary processing uses only the presence of high frequency power contained in pulses with a standardized duration equal to 450 ns separated from one another by a standardized spacing that is a multiple of 1.45 .mu.s. Each pulse conveys a binary information element. Consequently, if its power goes beyond a certain threshold which is a function of the reference pulses F1 or F2, this information will be equal, for example, to 1. If not, it will be equal to 0. The power sent out by the transponders of the aircraft is generally transposed, at reception, into the range of the so-called intermediate frequencies, typically of the order of 60 MHz, and then detected through logarithmic limiter amplifiers aimed notably at absorbing the high dynamic range of power received and at preventing, for example, the saturation of the processing circuits.
In the event of garbling between two pulses received by a secondary radar, the prior art methods of analysis, notably the systems used to analyze the power of the signal Q.SIGMA., do not reveal the existence of two intermingled (hence garbled) pulses when the difference in power between these pulses is below 6 dB for example, i.e. when the power of one of them is not at least equal to twice that of the other one. Errors of detection are then made in the duration and position of the two garbled pulses.
There are existing approaches that can be used to resolve these problems. In particular it is possible, should the reception antenna of the secondary radar be of the monopulse type, to make use of power and difference measurement signals coming from the sum and difference channels. The power signal is, for example, the logarithmic expression of the signal coming from the sum channel. This signal may be referenced Log .SIGMA.(t). The difference measurement signal is the ratio of the signal of the difference channel to the signal of the sum channel, it may be referenced .DELTA./.SIGMA.(t). These approaches analyze the shape of the signals Log .SIGMA.(t) and .DELTA./.SIGMA.(t). These are pulsed signals, and are synchronous with the received pulses. According to these approaches, if there are ripples that are superimposed on the signals Log .SIGMA.(t) and .DELTA./.SIGMA.(t) and that go beyond a certain threshold, then the presence of garbling is deduced. These ripples are actually due to the variations in operating frequency between the different transponders sending their responses out towards the secondary radar. Now, if these variations are too small, then these ripples cannot be detected. Typically, such a variation should be greater than about 1 MHz. However, the modern digital transponders, which are based on quartz oscillators with a precision of less than .+-.10.sup.-5 work with frequencies that are increasingly close to each other and do not allow these approaches to be used. Indeed, if the ripples due to garbling are at a frequency of the order of 20 kHz, which may be the case with these modern transponders, with the received pulses lasting 450 ns and the period of the ripples lasting 50 .mu.s, then it will be impossible to detect any variation in level or in variation measurement within the received pulse.